Technical Field
The present disclosure relates to a battery monitoring apparatus and a wireless field device having the battery monitoring apparatus, more particularly, to a battery monitoring apparatus suitable for assessing the life of a battery in an electronic device driven by a primary battery.
Related Art
Recent process control systems are sometimes configured using wireless field devices serving as a kind of wireless communication apparatus. As a driving power source for these wireless field devices, a primary battery (hereafter sometimes simply referred to as a battery) is used which is typified by, for example, a thionyl chloride lithium ion battery featuring low self discharge and being capable of supplying a nearly flat, stable and high output voltage (3.6 V) for a long time.
Process control systems for electricity, gas, and water supply and sewerage, for example, are desired to operate continuously for a long time without stopping except for periodic maintenance/inspection periods. For this reason, a sudden operation stop of a wireless field device due to reaching the life of a drive battery must be prevented from occurring as much as possible.
Although the primary battery being used as the driving power source of the wireless field device can supply a stable output voltage for a relatively long time, the consumption current of the battery differs significantly depending on the operating conditions of the wireless field device individually set by the user, whereby it is difficult to manage the life of the battery in a single uniform way.
Hence, in a related-art wireless field device as an electronic device, for example, a battery monitoring apparatus (battery monitor 5) having such a circuit configuration as shown in FIG. 6 is built therein. In FIG. 6, a primary battery 1 delivering an output voltage V0 is connected in parallel with a voltage measurement section 2 via an internal resistance R0 being connected in series, and one terminal of a current measurement section 3 is connected to one terminal of the voltage measurement section 2.
A load circuit 4 is connected to the other terminal of the current measurement section 3 and to the other terminal of the voltage measurement section 2. The load circuit 4 includes a load ZL and a series circuit of a resistor R1 and a switch SW1 connected across both the terminals of the load ZL.
The battery monitor 5, having such function blocks as shown in FIG. 7, turns on/off the switch SW1 of the load circuit 4 depending on the cycle of measurement, controls the measurement operations of the voltage measurement section 2 and the current measurement section 3, obtains the measurement data of the respective measurement sections, and executes such a battery life assessment process as shown in FIG. 8.
In FIG. 7, a measurement cycle controller 5a, being used to control the measurement cycle of the entire circuit, outputs predetermined timing clocks to a switch drive controller 5b for turning on/off the switch SW1, to a voltage measurement controller 5c for controlling the voltage measurement operation of the voltage measurement section 2, and to a current measurement controller 5d for controlling the current measurement operation of the current measurement section 3, so as to obtain voltage and current measurement data, for example, once per hour, the timing clocks being synchronous with one another.
The voltage measurement data V2 of the voltage measurement section 2 in the ON state of the switch SW1 is stored in a voltage measurement value memory 5e via the voltage measurement controller 5c. Similarly, the current measurement data I2 of the current measurement section 3 in the ON state of the switch SW1 is also stored in a current measurement value memory 5h via the current measurement controller 5d as necessary.
When the switch SW1 is turned on, a certain level of load current I1 flows from the battery 1 to the series circuit of the resistor R1 and the switch SW1. The life of the battery 1 can be detected promptly by measuring the voltage in the circuit.
The voltage measurement data V2 stored in the voltage measurement value memory 5e is input to one input terminal of a voltage comparator 5g, and the voltage reference data Vr stored in a voltage reference value memory 5f is input to the other input terminal of the voltage comparator 5g. 
The initial open circuit voltage of the battery 1, the previous measurement voltage or the like is used as the reference voltage Vr that is used for comparison. The necessity of generating an alarm is judged depending on how much voltage drop occurred from the reference voltage Vr.
The current measurement data I2 stored in the current measurement value memory 5h is input to one input terminal of a current comparator 5j, and the current reference data Ir stored in a current reference value memory 5i is input to the other input terminal of the current comparator 5j. 
The voltage comparator 5g compares the voltage measurement data V2 read from the voltage measurement value memory 5e with the voltage reference data Vr read from the voltage reference value memory 5f. If the voltage measurement data V2 is lower than the voltage reference data Vr (V2<Vr), the voltage comparator 5g instructs an alarm output module 5k to output a predetermined alarm.
The current comparator 5j also compares the current measurement data I2 read from the current measurement value memory 5h with the current reference data Ir read from the current reference value memory 5i. If the current measurement data I2 read from the current measurement value memory 5h is lower than the current reference data Ir read from the current reference value memory 5i (I2<Ir), the current comparator 5j instructs the alarm output module 5k to output a predetermined alarm.
FIG. 8 is a flowchart showing the flow of a process for assessing the life of the battery 1 on the basis of the output voltage in the circuits shown in FIGS. 6 and 7. The switch SW1 is turned on and the output voltage V2 of the primary battery 1 is measured by the voltage measurement section 2 (at step S1), and the output voltage is stored in the voltage measurement value memory 5e. 
The voltage comparator 5g compares the voltage measurement data V2 with the voltage reference data Vr and judges whether V2<Vr is established (at step S2).
In the case that V2<Vr is established, the voltage comparator 5g instructs the alarm output module 5k to output a predetermined alarm, such as sound, light, electronic mail, etc. for externally notifying that the output voltage of the battery 1 has dropped, whereby the alarm output module 5k outputs the predetermined alarm (at step S3).
On the other hand, in the case that V2<Vr is not established, a sufficient output voltage is obtained. Hence, no alarm is output and the life assessment process is ended.
Patent Document 1 discloses a technology in which the charging state of a nickel-cadmium battery or a nickel-hydrogen battery is detected without using a temperature sensor and the completion of charging is detected accurately.
Patent Document 2 discloses a technology in which the voltage value of a battery during discharge is calculated using the internal resistance value of the battery and the reference resistance value of a load, whereby errors in the calculation values of SOH and SOC are reduced and the state of the battery can be estimated properly.